Guide rollers are used, in particular, at the output side of a rotary printing press if it is necessary to reroute several partial webs into which a paper web imprinted in the rotary printing press has been cut and which partial webs leave the press, lying next to each other, to a cutting device, in such a way that the partial webs can be placed on top of each other, folded, if necessary, and further processed.
DE 34 36 870 C1 discloses a turning bar having an axially displaceable piston.
Turning bars, which are configured with an interior which can be supplied with compressed air, are known from U.S. Pat. No. 5,464,143 A. The jacket of the turning bar is provided with holes over at least a part of its circumference. Air can escape through these holes from the interior of the turning bar in order to form an air cushion between the turning bar and the web of material.
Pistons can be axially displaced in the interior of these turning bars which pistons, depending on their position inside the turning bar, will block a greater or fewer number of holes. This is done in order to match the width of the air cushion generated on the turning bar to the width of the web of material to be rerouted.
Such a matching of width is necessary, because compressed air escaping through the holes in the turning bar shell that is not covered by the web of material leads to an undesirable weakening of the air cushion which is generated by the holes covered by the web of material. As a result, the effect of the air cushion can be insufficient in the case of a narrow web of material. Moreover, the air currents exiting through the uncovered holes can interfere with the running of adjacent webs of material.
By using the turning bar disclosed in the above mentioned document, it is not possible to achieve an optimum matching of the shape of the air cushion to the course of the web of material. The reason for this is that the piston interiors are planar, while the edge of a web of material on the shell of the turning bar, which web of material typically is to be deflected by 90°, follows a helical line. If the piston is set in such a way that all of the holes which are not covered by the web of material are blocked, triangular zones are created in the edge areas of the web of material, in which zones the material web is not completely supported by an air cushion. If the pistons are set in such a way that the entire web is supported by an air cushion, holes which are supplied with compressed air necessarily remain uncovered.
To avoid this problem, pistons are used whose side facing the interior of the turning bar is not planar, but instead has edges whose courses respectively correspond on one half of the circumference to a right-turning helical line, and on the other half to a left-turning helical line. The pistons can be rotated around their axes within the turning bar so that, depending on the direction in which the web of material is rerouted, one or the other of the two helically-shaped edges can be turned toward the perforated portion of the circumference of the outside of the turning bar around which the web of material to be rerouted is looped.
By use of a piston shaped in this way it is possible to optimally use the compressed air available. The web of material is homogeneously supported by an air cushion over its entire width, and the escape of air which flows from uncovered holes is prevented.
In order to make use of the advantages of the piston described in the above mentioned U.S. patent, as well as those of the piston described here, every time the turning bar is used for rerouting a web of material of a different width, it is necessary to match the position of the pistons exactly to this material web width. Even if, instead of the width of material being changed, only the direction of rerouting the web of material is changed, this change can require a repositioning of the pistons. This process is very labor-intensive and arduous, because the pistons are not visible inside the shell of the turning bar. Although an operator can possibly look through the holes in the turning bar, as long as no web is conducted over the turning bar, in order to determine whether or not a hole is blocked by a piston, as soon as the web is being conducted over the turning bar, there is no longer an opportunity to check whether all holes covered by the web are indeed free, so that the air cushion is, in fact, generated over the required width.
JP 11-246098 A describes a guide roller which can be charged with air and whose jacket is provided with holes. These holes can be selectively closed off as a function of the web width by the use of a blocking element, which element is hydraulically displaceable in the interior chamber. Sensors are arranged for detecting the web width.